This invention relates to a dry etching system and a dry etching method used for fine fabrication of semiconductor devices and, more in particular, it relates to a dry etching system and a dry etching method for attaining highly accurate dry etching fabrication for insulation films.
In a semiconductor device, for electrically connecting transistors formed on a wafer and metal wirings, as well as connection between metal wirings, contact holes are formed to a insulation film formed on a transistor structure and between wirings (thin film mainly composed of SiO2, hereinafter referred to as an oxide film) and electric conductors are filled-in the contact holes. In the dry etching, etching gas is introduced into a vacuum chamber, a high frequency bias or xcexc wave is applied to the gas to generate plasmas, the oxide films are etched selectively by active species and ions formed in the plasmas to form contact holes. Upon etching, a resist thin film transferred with a hole pattern is formed on the oxide film.
In the contact hole fabrication, the oxide film has to be etched selectively to silicon forming a resist film, a wiring layer below the contact hole, or insulation film different from the etched film.
For the fabrication of the contact hole, etching is conducted by introducing fluoro carbon gas such as CF4, CHF3, C4F8, C4F6 or C5F8 and Ar gas into an etching device, conducting high frequency wave plasma discharge under a gas pressure condition of from 0.5 Pa to 10 Pa and under the condition of applying a Vpp voltage from 0.5 to 2.0 kV to the wafer. In a case where the thickness of the oxide film between the wiring layers is large and the aspect ratio of the contact hole (depth/diameter) is high, oxygen gas and CO gas are added for improving the hole penetration and addition of the CO gas has an effect of further enhancing the selectivity to the resist and the nitride film
In recent years, Cu is used as a wiring material for higher speed operation of semiconductor devices, and use of organic insulation film and organic silicon oxide film as the insulation film between the wirings has been considered. Nitrogen-containing gas is used for the organic insulation film and gas substantially identical with that for the oxide film is used for an organic silicon oxide film.
A technique of forming contact holes at high aspect ratio to an oxide film using polycrystal Si as a mask is disclosed in Jpn. J. Appl. Phys. Vol. 36 (1997), pp 2470-2476.
However, in a case of fabricating contact holes at high aspect ratio in the existent etching systems, oxygen gas or CO gas has to be added in excess in order to avoid the problem that etching is interrupted failing to form an opening. Since CO constitutes a source for supplying oxygen radicals, etching has been conducted under oxygen radical rich condition. However, as the oxygen radicals become excessive, deposition films are not formed on the lateral side in the upper portion of the hole, and the hole fabricated shape is enlarged by the incidence of ions scattered by a mask. Extension of the hole occurs at a position somewhat deeper than the hole opening (upper portion) Referring more specifically, a hole diameter is enlarged compared with the hole opening and the hole-diameter is smaller as the hole depth is greater. That is, side etching occurs in the midway of the hole in the fabricated shape. Such a phenomenon that the hole diameter is enlarged is referred to as bowing.
When bowing should occur, gaps are formed when electroconductive materials such as polycrystal silicon or tungsten is filled in the hole to cause failure in semiconductor devices. Since the bowing becomes conspicuous as the hole aspect ratio is higher, it results in a bar for the size-reduction of semiconductor devices. Particularly, in the oxide film etching, bowing appears at an aspect ratio of 6 or more, and the bowing increases as the aspect ratio increases. The size of semiconductors has been reduced and fabrication at an aspect ratio of 10 or more is necessary but size reduction becomes difficult in view of trade off with the bowing shape.
The bowing phenomenon also occurs for the materials other than the oxide film and it becomes conspicuous, particularly, in organic insulation films. Depending on the condition, the bowing is conspicuous in the organic insulation films and organic silicon oxide films at an aspect ratio of about 2.5 or more. Since such thin films are used as insulation films between wirings, the aspect ratio is about from 5 to 10 in fine portions.
In order to prevent bowing, depositing gas is sometimes added but this results in a side effect such as lowering of the etching rate.
A problem to be solved by this invention is to suppress radicals causing bowing during etching, thereby reducing bowing to attain fine fabrication for insulation films.
In the case of etching for oxide films by C4F8 gas and CO gas, CF2, F, O and C are formed mainly due to dissociation in plasmas (the term xe2x80x9cradicalxe2x80x9d is sometimes attached at the head of names of atoms and molecules but this indicates same materials). In addition, while CF3, CF, C2F4, C3F7 are also formed, explanations therefor are to be omitted since they have no influence in the aspect of this invention. The sticking coefficient of CF2, F, O and C on the lateral surface of a hole is represented as: SC greater than SF≈SO greater than SCF2 (SC, SF, SO, SCF2 represents respectively sticking coefficients for C, F, O and CF2). For the sake of convenience, F or O is also represented by the sticking coefficient and the sticking coefficient corresponds to the etching probability for the deposition film. FIG. 1 is a schematic view for determining the dependence hole aspect ratio of the relative wall deposition in the mixed gas process of Ar, C4F8, O2, CO using the foregoing relation of sticking coefficients. Depending on the relation of the sticking coefficients, C can be a protective film to a mask but, as shown in a curve 102, the reaching amount into the hole decreases abruptly as the aspect ratio of the hole becomes higher. On the other hand, as shown in a curve 103, it scarcely decreases even if the aspect ratio of the hole increases. On the contrary, since F and O react with depositing radicals C and CF2 in the upper portion of the hole, the reaching amount to the vicinity of the bottom is decreased in a case of a hole at a high aspect ratio as shown by a curve 101.
For fabricating a hole at a high aspect ratio, it is necessary to supply a great amount of C and F radicals in order to suppress interruption of etching caused by excessive CF2. Therefore, O and F radicals become excessive in a shallow portion of the hole and side wall deposition film with C and CF2 is scarcely formed. Therefore, when ions scattered in the upper portion of the hole are entered to the lateral surface of the hole, SiO2 film is etched (or sputtered) to bring about bowing. Assuming that the deposition film 2 formed of C and CF2 is etched by F and O, a dimension shift for the fabricated shape occurs as shown by a curve 201 in FIG. 2. In this case, negative size shift means bowing. Bowing occurs at an aspect ratio of about 3.5.
In a case where holes are etched under the same condition (hereinafter referred to as uncontrolled etching), it is necessary to correspond the amount of F and O radicals to the hole at the highest aspect ratio. However, since the hole aspect ratio increases along with the progress of etching, an aspect ratio is about 3 for the diameter of 0.2 xcexcm or less at the initial stage of etching even if the resist film thickness is considered. Since the resist film thickness is reduced as the size reduction of the devices progresses, the aspect ratio at the initial stage does not change so much depending on the generation of the semiconductor devices. As has been described above, since the aspect ratio at the initial stage is small, F and O are excessive at the initial stage of the etching in uncontrolled etching.
With the reasons described above, if the O2 flow rate and the CO flow rate are increased along with etching time, bowing can be reduced. FIG. 2 shows a dimension shift of a fabricated shape by a curve 202 in a case of increasing the O2 flow rate from 0 ml/min to 8 ml/min and the Co flow rate from 50 ml/min to 120 ml/min in proportion with time for the dimension shift of the fabricated shape. Bowing is remarkably reduced by decreasing excessive O as described above.
However, if the flow rate is merely changed with time, since response to the occurrence of interruption of etching and the flow rate is slow depending on the state of the system, the control method depends on the length of the pipeline and the type of the flow rate controller. As a means for coping with such problems, it is effective to adopt a method of monitoring the progress of oxide film etching and controlling the flow rate based on the monitored data, a method of using those with short response time such as application of bias to eliminate excessive F and C and, further, means for combining them. As a method of monitoring the progress of the etching, there can be mentioned emission spectrum of plasmas, optical film thickness gage, mass flow analyzer, laser induced optical spectrum and the like. Furthermore, in a case capable of controlling two or more electron temperatures regions, there is a means for controlling the dissociation of F by the control for the width of the electron temperature regions.
In the organic silicon film and the organic insulation film, bowing occurs due to excess O, F or Cl, as well as bowing also occurs due to excessive N radical. Accordingly, bowing can be reduced by controlling any of excessive radicals of O, F, Cl and N.
This invention provides an etching method of controlling the incident amount of O and F radicals along with lapse of etching time in an oxide film etching, thereby capable of suppressing bowing and obtaining a fabricated shape which is vertical and at high aspect ratio. In the etching of insulation films including organic materials, the invention provides an etching method capable of obtaining fabricated shape which is vertical and at a higher aspect ratio by adjusting the incident amount of N radicals.